1. Field of the Invention
This invention relates to ultrafine fiber of an ethylene tetrafluoride copolymer having a high melting point. More particularly, it relates to fiber having an orientation degree (.pi.) of at least 0.6 and an averae fineness of 0.0001 to 0.9 denier. This invention is also concerned with a process for producing such fiber or a porous membrane thereof by treating with a solvent fiber of the type composed of an ethylene tetrafluoride copolymer as an island component and a polyolefin as an ocean component.
2. Description of the Prior Art
It is known that a copolymer consisting mainly of ethylene tetrafluoride is excellent in various properties, including resistance to heat and chemicals, electrical insulating property, water or oil repelling property and mechanical properties. Ultrafine fiber of such a copolymer having a high degree of orientation or a porous membrane thereof is expected to be useful for a wide variety of industrial applications. It has, however, been very difficult to produce ultrafine fiber having an average fineness of 0.0001 to 0.9 denier and a high degree of orientation, since these copolymers have a high melting point and are not satisfactory in workability.
It is known that ulrafine fiber can be produced from, for example, a polyester or nylon, if it is extruded with, for example, polystyrene to form fiber of the type in which it defines an island component, while polystyrene constitutes an ocean component, and if the polystyrene is removed by a solvent, as disclosed in Japanese Unexamined Patent Specification No. 114773/1977. This method is applicable to a fiber-forming polymer such as a polyester or nylon, and based on the possibility of a chemical separation for a combination of a fiber-forming polymer having a high intermolecular cohesive force and soluble in a polar solvent and a polymer soluble in a nonpolar solvent, such as polystyrene. It is easy to find a thermally stable polymer which can be extruded with a known fiber-forming polymer, such as a polyester or nylon, since the extrusion of any such fiber-forming polymer does not require a very high temperature. The high intermolecular cohesive force of the fiber-forming polymer enables oneself to flow for satisfactory orientation during its extrusion with another polymer, such as polystyrene, and facilitates the formation of ultrafine fiber if it is given an ultrafine fibrous shape.
U.S. Pat. No. 3,099,067 discloses the production of ultrafine fiber of polytrifluorochloroethylene by removing polyethylene from mixed fiber containing polytrifluorochloroethylene as an island component and polyethylene as an ocean component. A copolymer consisting mainly of perfluoroethylene, such as ethylene tetrafluoride, however, has a high melting point and involves, therefore, a lot of difficulty in extrusion. The mere spinning of its mixture with polyethylene having a large melt index does not produce ultrafine fiber having a high degree of orientation. An ethylene tetrafluoride copolymer has a low intermolecular cohesive force, as opposed to the so-called fiber-forming polymer. Accordingly, it is very likely that, even if an apparently fibrous material may be formed, when it may not have a sufficiently high degree of orientation, it may not be useful fiber. Therefore, there has been no ultrafine fiber of ethylene tetrafluoride copolymer having a high degree of orientation.
A porous membrane of a fluorine-containing resin can be produced by various methods, for example:
(1) Dissolving a resin in a solvent, gelling its solution by cooling or otherwise, and removing the solvent (gelation method); PA1 (2) Molding a mixture of a resin and a removable substance and removing the removable substance (mixing method); PA1 (3) Stretching a molded resin product which is not homogeneous, for example, which has undergone crystallization or contains foreign matter (stretching method); or PA1 (4) Sintering the particles or fiber of a resin (sintering method).
The gelation method is, however, unsuitable for a copolymer of ethylene tetrafluoride, since an appropriate solvent is not easily available. As it is a relatively soft resin, the sintering method is also unsuitable as failing to form pores. The stretching or mixing method is, therefore, usually employed. The stretching method has, however, the disadvantage of forming an anisotropic membrane which is stiff and easy to tear. The production of a large membrane requires a large apparatus. The mixing method has the advantage of producing a fine porous membrane which is similar to paper or leather, but the disadvantage of forming a laminar product having a very low degree of permeability if a large molding machine, such as an extruder, is employed. This method is, therefore, used for only a limited scope of industrial application. In order to overcome this disadvantage, there has been proposed a method which comprises kneading a mixture of a thermoplastic resin and a fluorine-containing resin at a temperature below their melting points to effect its fibrilation and removing the thermoplastic resin, as disclosed in Japanese Patent Publication No. 8506/1974. If a fine powder is employed, this method enables the production of a membrane composed of very fine fibril, but as its kneading proceeds, its viscosity increases to the extent that it is practically impossible to continue kneading and obtain a dense membrane. Another disadvantage of this method is that the production of a large membrane requires a large apparatus.
A conventionally available fiber of an ethylene tetrafluoride copolymer having a diameter of several tens to hundreds of microns was relatively soft and difficult to handle. And if this fiber may be intertwined, due to its large diameter, the formation of a membrane having fine pores was difficult.